1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems for generating, acquiring and processing seismic data and, more particularly, to mechanisms and techniques for separating recorded seismic data simultaneously generated by plural seismic sources.
2. Discussion of the Background
Seismic data acquisition and processing may be used to generate a profile (image) of geophysical structures under the ground (subsurface). While this profile does not provide an accurate location for oil and gas reservoirs, it suggests, to those trained in the field, the presence or absence of such reservoirs. Thus, providing a high-resolution image of the subsurface is important, for example, to those who need to determine where the oil and gas reservoirs are located.
In the past, conventional land seismic acquisition generally employed multiple vibrators (seismic sources) acting one at a time. In land-based operations, the vibrators are positioned at a source location and then actuated. Once activated, the vibrators generate a sweep that typically lasts between five and twenty seconds and typically spans a predetermined range of frequencies. A recording system that is connected to a plurality of receivers, typically geophones for land-based seismic exploration, is employed to receive and record the response data. For reflection seismology, the record length is typically set to equal the sweep length plus a listen time equal to the two-way travel time, which is the time required for the seismic energy to propagate from the source through the earth to the deepest reflector of interest and back to the receiver. The vibrators are then moved to a new source location and the process is repeated.
For marine seismic acquisition, traditionally a vessel tows plural streamers having multiple seismic receivers configured to record seismic data. The vessel also tows a seismic source that imparts energy into the water. The seismic energy travels toward the subsurface and is partially reflected back to the sea surface. The seismic recorders record the reflected seismic waves.
When the source (either land source or marine source) is fired with standard data acquisition, the subsequent recording time is defined so that all useful reflected/diffracted energy is recorded before the next shot fires. This delay time imposes constraints on the acquisition rate and, hence, increases the cost of acquisition.
To reduce the acquisition time, it is possible to simultaneously shoot the sources. Acquisition of simultaneous source data means that the signals from two or more sources interfere at least for part of the record. By acquiring data in this way, the time taken to shoot a dataset is reduced along with the acquisition costs. As an alternative to reducing the acquisition time, a higher density dataset may be acquired in the same time. For such data to be useful, it is necessary to develop processing algorithms to handle source interference (cross-talk noise).
Source interference appears because subsurface reflections from an early source excitation may be comingled with those that have been sourced later, i.e., a “blended source” survey is acquired. Note that this is in contrast to conventional surveying techniques, wherein the returning subsurface reflections from one source are not allowed to overlap with the reflections of another source. Although the blended-source approach has the potential to reduce the time in the field, thereby proportionally reducing the cost of the survey, one problem is that it can be difficult to separate the individual shots thereafter. In other words, what is needed in interpreting seismic data is the depth of each reflector, and the depth of a reflector is determined by reference to its two-way seismic travel time. Thus, in a multiple-source survey it is the goal to determine which of the observed subsurface reflections is associated with each source, i.e., to de-blend the data; otherwise, its two-wave travel time cannot be reliably determined.
In this regard, FIG. 1A shows sources being actuated at different spatial positions 10, 12 and 14 with delay times such that the recorded wavelets 10a-c corresponding to spatial position 10 do not interfere (in time) with wavelets 12a-c corresponding to spatial position 12. The signal recorded at the receiver can be considered as a continuous recording (16) or separated to form regular seismic traces for each individual shot as shown in FIG. 1B. The traces as illustrated in FIG. 1B form a receiver gather 20. Each trace in the receiver gather 20 relates to a different shot and has a different position on axis X, and each wavelet has a different time on a temporal axis t.
FIG. 2A shows a similar source configuration as in FIG. 1A, but now the sources are simultaneously activated so that, for example, the wavelet 10c might be superposed (in time) with the wavelet 12a. FIG. 2B shows the receiver gather 30 formed through pseudo-de-blending. Pseudo-deblending involves forming regular seismic traces from the continuous recording based on the start time of the actuation of each shot with no attempt to mitigate cross-talk noise. The data of FIG. 2B has been shot in less time than the data in FIG. 1B, but cross-talk 32 is observed and noise on one trace is signal on another trace.
Thus, for the gather 30 in FIG. 2B, it is necessary to separate the energy associated with each source (de-blend) as a preprocessing step, and then to proceed with conventional processing. To make the separation easier, it is generally advantageous to use a variety of different source signals, for example, different vibroseis sweeps or pseudo-random sweeps. When energy from a given source is correlated with the sweep signal, this results in a focusing of the energy of that source while keeping energy from other sources dispersed. The actual timing of the shots may also be used to successfully de-blend the energy from the sources.
The randomized timing of source actuation gives rise to a randomness in timing of the cross-talk noise in all domains other than the shot domain. For example, FIG. 3 (corresponding to Hampson et al., Acquisition using simultaneous sources, Leading Edge, Vol. 27, No. 7, the entire content of which is incorporated herein by reference) shows the same recorded seismic data from a marine simultaneous shooting dataset in different domains, i.e., common shot, common receiver, common midpoint, common offset.
Traditionally, de-blending of simultaneous shooting data falls into the following three categories, all of which rely on some degree of randomized shooting. The first category is impulsive de-noising. This method (disclosed for example by Stefani et al., Acquisition using simultaneous sources, 69th EAGE Conference & Exhibition, the entire content of which is incorporated herein by reference) uses the fact that when data is sorted into any domain other than the common shot, the cross-talk noise from other sources has random timing as illustrated in FIG. 3. It is noted that in the common shot domain, the cross-talk noise 40 is continuous. This random timing allows the use of impulsive-noise attenuation techniques which are already available and used in other processing steps, for example, swell-noise attenuation. While this method can be effective for removing the strongest cross-talk energy, low amplitude cross-talk noise is not seen as impulsive and will not be removed.
A second category includes the iterative coherency enhancement/de-noising. Iterative coherency enhancement/de-noising techniques are described in, e.g., Abma et al., Separating simultaneous sources by inversion, 71st EAGE Conference & Exhibition, the entire content of which is incorporated herein by reference, and rely on the fact that cross-talk noise on some traces is a duplication of signal energy on other traces. This means that with the knowledge of the timing of all shots, a signal estimate made for one source can then be used to reduce the level of cross-talk for all other sources.
A third category includes the full modeling of energy from all sources. The full modeling scheme (e.g., Akerberg et al., Simultaneous source separation by sparse Radon transform, 78th Ann. Internat. Mtg.: Soc. of Expl. Geophys, and Moore et al., Simultaneous source separation using dithered sources, 78th Ann. Internat. Mtg.: Soc. of Expl. Geophys, the entire contents of which are incorporated herein by reference) has similarities to the iterative de-noising method, except that this formulation solves the relationship between source energy and cross-talk noise implicitly at the core of the problem formulation. The equations can be formulated as designing a transform domain for each source or spatial area (e.g., tau-p domain, Fourier domain, etc.) such that when it is reverse-transformed and re-blended, the raw input data is reconstructed as accurately as possible in a least squares sense.
This technology has the timings and positioning of all sources at the core of the algorithm and also relies on a sparse solution to the equations. Once the transform domains have been calculated, the final step to de-blend the data requires application of reverse-transform without re-blending. While this method may result in some filtering of the original data, it removes low amplitude cross-talk noise and preserves the primary signal. This method could be considered to be an alternate way of solving the same problem as the iterative coherency enhancement/de-noising technique (with the analogue of sparse least squares Radon versus inversion through “iterative cleaning”).
One weakness known in the art of the full modeling approach is that having one transform per source requires that sources must be traveling in a controlled way. While this may be satisfactory for marine acquisition where the boat speed is substantially constant, this is not sufficient for many 3D land blended acquisition patterns where the vibroseis trucks shoot in a less controlled way, sometimes in a random way. While for some land shooting patterns it would be possible to have one transform per source (if the source fired only within a fixed rectangle), the resulting data would be undesirable due to edge effects in the receiver gather at the edge of each source. For this reason, to process blended land acquisition, it is necessary to extend the approach so it is not limited to one transform per source.
Thus, there is a need to develop a method capable of processing blended seismic data while not being limited as noted above.